1. Field of the Invention
The present invention relates to an optical disc apparatus using a recordable optical disc, in particular, to an optical disc apparatus corresponding to ALP (Automatic Laser Power Control).
2. Description of the Related Art
Recordable optical discs such as magneto optical (MO) discs and phase-change type optical discs have been widely used. In these optical discs, when data is recorded and reproduced thereto and therefrom, a tracking control is required so as to trace the laser spot to a predetermined position. A sample format in which servo areas for tracking control are pre-recorded with predetermined intervals in track direction is known. In a servo zone, two pits are formed radially in opposite directions against the center of a track. The pits are referred to as wobble pits. The tracking control is performed so that the center of the laser spot accords with the center of the track corresponding to the levels of the reproduced signals of the two pits.
When the optical disc is rotated with a constant angular velocity (CAV), the linear velocity is reverse proportional to the distance from the center of the disc. When the data rate of record data is constant, the record density in the outer areas decreases. To prevent this problem, a zone record system has been proposed. In the zone record system, the disc is rotated at the CAV. The record area on the disc is radially divided into a plurality of sub-areas (referred to as zones). The record density of each zone is almost kept constant.
In other words, in outer zones of the disc, the data rate of record data increases. In the innermost zone, the frequency of the data clock (referred to as the channel clock) that synchronizes with the data rate is the minimum. In the outermost zone, the frequency of the data clock is the maximum. In each zone, the frequency of the data clock is constant. The sample format can be applied to the zone record system. In this case, the time intervals in the servo areas are constant in not only the zones, but the entire record areas.
In the magnetic field modulating type MO disc recording apparatus, when data is recorded, the laser light is turned on and off depending on the rate of the data clock. When data is reproduced, the laser light is successively emitted. When data is recorded and reproduced, the laser power is controlled in the following manner. When data is reproduced, the power of the laser beam than is successively emitted is controlled according to the level of the light reflected from the servo area. When data is recorded, the power of the laser is controlled according to the level of light reflected from a power control area (referred to as ALPC (Automatic Laser Power Control) area) formed on the disc.
In the above-described zone record system, since the frequency of the data clock varies, zone by zone, the frequency of the emitted pulse light that synchronizes with the data clock varies, zone by zone. When the frequency of the emitted pulse light varies, the power of the recording laser is properly adjusted by the ALPC. Otherwise, pits with a required size (actually, the pits are magnetic patterns) cannot be formed on the disc. Generally, the laser power should be increased according to the clock frequency.
In the zone record system, when the spot of the record laser beam is moved to another zone, until an optimum laser power thereof is set, a time lag takes place to some extent. When a large amount of data such as image data is recorded, it may be recorded in two or more successive zones. When the zone is changed, if there is a time lag, data cannot be successively recorded. Alternatively, when the zone is changed, a special data process may be required. Thus, the process or hardware becomes complicated.